Boolean Modifier Issue
Its not good to stack a lot of un-applied modifiers when you are using booleans - they are temperamental and it is better to apply the mods at every step. I have applied all of your mods in the order they appear up to the boolean operation and the phone case has 1104 zero faces and 48 zero edges created in the steps you have taken. You can remove these by selecting all in edit then using mesh/cleanup/degenerate dissolve and then your booleans will work better. Any work involving booleans benefits massively from enabling the in-built 3D printing add on in preferences.